1. Field of the Invention
The invention relates generally to an economically and environmentally efficient recovery method for continuous calcium extraction; precipitated calcium carbonate production (PCC); and simultaneous iron enrichment which consumes industrial waste products and green house gases as starting materials.
2. Background of the Invention
The increasing atmospheric concentration of CO2 attributed to fossil fuel combustion is a serious problem, which contributes significantly to global warming. It is estimated that CO2 emission by the year 2100 will be approximately four times greater than that in 2000. Therefore, finding a practical method of reducing CO2 emissions is paramount. Methods have been proposed such as CO2 capture and sequestration (such methods include geological storage or ocean sequestration). However, currently proposed CO2 capture and separation processes are energy consuming and are the main reasons for the high cost of the sequestration process.
Coal and steel industries produce large volumes of ash and slag as industrial solid wastes, and flu gases that are rich in carbon dioxide. Slag is the partially vitreous by-product of smelting ore to separate a metal (usually iron) fraction, from the unwanted (siliceous) fraction. Slag is usually considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form, further the elemental constituents of slag will vary based on the geographic location from where the ore is mined. Slag from steel mills in ferrous smelting, mainly contains oxides of calcium, silicon, magnesium, and aluminum. Any sandy component or quartz component of the original ore automatically carries through the smelting process as silicon dioxide.
Once smelting is complete, the slag is typically channeled out of the furnace, and flash cooled with water. This rapid cooling, often from a temperature of around 2,600° F. (1,430° C.), comprises the start of the slag granulating process. Water then carries the slag as slurry to agitation tanks. The filter beds then retain the slag granules, which may be ground further. Typically, slag will be recycled until it is depleted of iron, but extraction process limitations means that a residual amount of iron (10%), is still present in the slag. The slag also contains about 40% calcium, typically in the form of calcium silicates.
While the residual iron is valuable to the steel industry, the cost of further extraction and recovery is prohibitive. For example, prior art methods include grinding the slag to form ultra fine particles, thereby freeing the bound iron, which can be removed magnetically; however the electrical cost of mechanically grinding the slag negates the value of the iron recovered. The calcium rich slag granules, calcium rich slag fines or combinations thereof, are then typically stored in landfill.
Evidently, in the U.S. alone, these industries produce many millions of tons annually of solid waste (which has very little economic value) and green house gases that pose a serious environmental impact.
Accordingly, there remains a need in the art for a method of reducing such waste that is both economically and environmentally viable. As such, embodiments described herein address the requirement for sequestration of environmentally harmful carbon dioxide by integrating carbon mineralization and iron recovery through slag refining, whereby the carbon mineralization produces high grade PCC.
In addition, this process yields near 100% conversion of such industrial wastes to environmentally stable and commercially valuable end products, by sequestering CO2. More over the process is itself environmentally a “sum zero system”, whereby all input chemicals that are not directly consumed in generating the reaction products approach 100% recycleable where in some embodiments chemical losses (weight % or molar) are due to limitations of the mechanical process control, or slippage, or entrainment, rather than a molar consumption due to chemical reactions]
Other objects and advantages of the invention will appear from the following description.